To analyze the relative roles of the mammary gland, kidney, and liver in the metabolic clearance of immunoreactive prolactin from the plasma of rats during the pregnancy and throughout lactation by the constant infusion to steady state method. To use metabolic clearance rates of prolactin plus endogenous prolactin levels, as measured by radioimmunoassay, to determine the secretion rate of prolactin into the circulation during late pregnancy and during lactation. To investigate with sensitive intramammary pressure techniques whether estrogen and progesterone exert either facilitatory and/or inhibitory influences at the mammary gland level upon the contractile response of the mammary gland myoepithelium. To analyze the role of the cerebral cortex and the sympathetic nervous system in the regulation of mammary contraction.